A navigation device for an automotive vehicle includes an angular speed sensor, an acceleration sensor and the like. The angular speed sensor detects angular speed of the vehicle around an axis perpendicular to a horizontal plane, on which the vehicle is disposed. The acceleration sensor detects acceleration of the vehicle in a front-back direction of the vehicle.
The navigation device is usually mounted on an instrument panel of the vehicle. When the device is tilted to the vehicle, a detection axis of the angular speed sensor or the acceleration sensor accommodated in the device is also tilted to a regular axis. Thus, the angular speed or the acceleration is not detected correctly.
In view of the above problem, a tilt angle of the sensor is detected even when the sensor is tilted to the regular axis, so that the angular speed or the acceleration is compensated by the detected tilt angle. Thus, driving conditions of the vehicle such as a driving direction and a slanting angle of the vehicle are compensated on the basis of detection signals from the angular speed sensor, the acceleration sensor and the like.
Specifically, for example, the acceleration sensor detects the acceleration in the front-back direction of the vehicle, so that the slanting angle of the vehicle (i.e., a pitch angle) in the front-back direction of the vehicle is detected on the basis of the detection signal from the acceleration sensor. In the navigation device including this acceleration sensor, change of the vehicle direction obtained on the basis of the detection signal from the angular speed sensor is compared with change of the vehicle direction obtained from change of vehicle position detected by a GPS receiver. A tilting angle of the navigation device to the vehicle in the front-back direction is calculated from a difference between the change obtained from the sensor and the change obtained from the GPS receiver. The pitch angle detected on the basis of the detection signal from the acceleration sensor is compensated on the basis of the calculated tilting angle. This compensation is disclosed in, for example, JP-A-2004-020207.
In the above technique, if the detection signal from the acceleration sensor or the angular speed sensor does not include noise, the pitch angle can detect correctly. This is because the pitch angle obtained from the detection signal of the acceleration sensor is compensated by the slanting angle of the navigation device obtained from the detection signal of the angular speed sensor.
However, in the above technique, it is assumed that the angular speed sensor and the acceleration sensor are tilted from an ideal detection axis. Therefore, the detection signal from the angular speed sensor includes an angular speed component around an axis perpendicular to a horizontal plane of the vehicle and a vibration component in a vertical direction of the vehicle. Further, the detection signal from the acceleration sensor includes an acceleration component in a front-back direction of the vehicle and a vibration component in the vertical direction of the vehicle. Thus, since each detection signal includes the vibration component, detection accuracies of the acceleration and the angular speed are reduced so that detection accuracies of a driving direction of the vehicle and a pitch angle are also reduced. Specifically, the detection signal from the acceleration sensor is correct when the actual detection axis of the acceleration sensor is parallel to the driving direction of the vehicle. The detection signal from the angular speed sensor is correct when the actual detection axis of the angular speed sensor is perpendicular to the horizontal plane of the vehicle. Therefore, since the navigation device is mounted to tilt from an ideal axis of the vehicle so that the actual detection axis of each sensor is tilted from the ideal axis, for example, a vibration noise caused by an up-down bounce of the vehicle is superimposed on the detection signal from each sensor when the vehicle rides over a step so that the vehicle bounces up and down. Accordingly, the detection accuracies of the acceleration and the turning angle are reduced.
The detection signals from the acceleration sensor and the angular speed sensor are filtered so that excess high frequency noise is removed. The filtering process is performed for example by a moving average method. The vibration noise having a low frequency such as a few Hz or lower cannot be removed by the above filtering process.
Further, when a low pass filter having a low cut off frequency for removing the vibration noise is introduced, the acceleration and the angular speed are not detected.
The above problem exists not only in the navigation device having the angular speed sensor and the acceleration sensor but also in another in-vehicle device having the angular speed sensor and/or the acceleration sensor. Here, the other in-vehicle device is, for example, a sensor for detecting a driving condition of the vehicle in order to control an engine of the vehicle, to control an attitude of the vehicle or to control a cruise of the vehicle.